140 TRANSMISSIVITY IN PASSIVE IRON WIRES 



pletely transmissive condition and conduct activation rapidly from 

 end to end. A region of decrement 4 cm. long is now made in each 

 of the long wires A and C as indicated ; this is done by pressing a piece 

 of platinum foil against the wire at a point 4 cm. from the bent end 

 and then touching this end with zinc ; the wire is thus activated from 

 its extremity to the edge of the platinum contact, and this recently 

 activated stretch (the shaded area in Fig. 2) acts as a region of decre- 

 ment for a period of several minutes. Wire B is used as a bridge 

 between wires A and C as indicated; it is left unchanged; i.e., is com- 

 pletely transmissive. When wires A and C are placed end to end, as 

 in Fig. 2, X, there is a continuous region of decrement ac 8 cm. long; 

 at a certain interval after the previous activation recovery in this 

 stretch has advanced so far that a wave started at the free end of 

 either wire will usually traverse ac for a distance of 6 to 7 cm.; i.e., 

 will cross the junction but fail to penetrate the whole region of decre- 

 ment. The time required to reach this condition in 70 per cent acid 

 is usually 6 to 7 minutes. The effect of interposing a completely trans- 

 missive region between wires A and C — after allowing the same time 

 for recovery as in the previous experiment, so as to have the same 

 degree of decrement as before — is obtained by uniting wires A and C 

 not directly but through the bridge of completely transmissive wire 

 B as indicated in Fig. 2, Y. In this case the wave, started at the 

 free end of A, typically passes along the whole three wires to the extrem- 

 ity of C. In favorable experiments the retardation in the decrement 

 region of A and the recovery of speed in the bridge B are plainly visible ; 

 the activation wave on entering C is then able to pass entirely through 

 its decrement area to the end. 



It is evident that the activation wave loses steadily in penetrative 

 power — or what might be called intensity^^ — ^s it passes along the 

 recently activated stretch or region of decrement; and regains this 

 power when it enters the completely transmissive stretch beyond. 

 This is the type of behavior also observed in a nerve fiber, as Adrian^^ 

 has shown. In the wire the variations in transmissive power are 



^^ Lucas points out that the only definite numerical measure, at present known, 

 of the "intensity" of a nerve impulse is the distance which it can travel along a 

 region of known decrement (Lucas, K., Conduction of the nervous impulse, New 

 York, 1917, Chapter II). 



